# Arrays without bounds

Regular old arrays have a size; you can't just have an infinite array. On the other hand, a lazy language such as Haskell does allow infinite lists. The idea behind the UnboundedArray module is to combine the O(1) access of arrays with the unbounded size of lazy lists.

```module UnboundedArray where
```

This data type is built on top of ordinary arrays and unsafe IO operations:

```import Data.Array
import Data.IORef
import System.IO.Unsafe
```

To keep things simple, an unbounded array is just a function from the natural numbers to array elements:

```type UnboundedArray a = Int -> a
```

I am just going to dump the code here instead of explaining it. The idea is to make an array and resize it when it becomes too small. If the size increases geometrically with each resize, then the amortized cost of a single access will be O(1).

```-- | Create an unbounded array from an infinite list
--   Accessing element /n/ takes /O(n)/ time, but only /O(1)/ amortized time.
unboundedArray :: [a] -> UnboundedArray a
unboundedArray xs = unsafePerformIO . unsafePerformIO (unboundedArrayIO xs)

unboundedArrayIO :: [a] -> IO (Int -> IO a)
unboundedArrayIO xs = do
theArray <- newIORef (listArray (0,0) xs)
return \$ \n -> do
let (0,size) = bounds ar
if n <= size
then return \$ ar ! n
else do let size' = max n (size * 3 `div` 2)
let ar' = listArray (0,size') xs
writeIORef theArray ar'
return \$ ar' ! n
```

So, what are UnboundedArrays good for? A simple application is memoization, for example:

```memoInt f = unboundedArray (map f [0..])

fib = memoInt realFib
where realFib 0 = 1
realFib 1 = 1
realFib n = fib (n - 1) + fib (n - 2)
```
```> map fib [1..20]
[1,2,3,5,8,13,21,34,55,89,144,233,377,610,987,1597,2584,4181,6765,10946]
```

But since we can use an arbitrary list for initialization the unboundedArray function can sometimes be more flexible/convenient than memoInt. Luke Palmer

That's a quaint little construction.

Now if only you could do something like that while maintaining some of list's GC properties. Twan van Laarhoven

Luke: Do you mean that you would like part of the structure to be collected if it is no longer used? The entire point of the UnboundedArray is that you can cheaply get any element that you accessed before.

Some optimization might be possible though. The current implementation keeps a copy of the entire list, but the first part of it is already stored in the array, so that is no longer needed. Then, if you know that you will never again need elements UnboundedArray a -> UnboundedArray a that destroys that. But, I don't think such an operation would be very useful in practice. Chao XuDate: 2013-03-29T00:46Z

Great work! You should post this into hackage, I can think of many times where I have to use this code.